I'm not seeing any downside of the latter. Taxes, maybe? Our custodial accounts were always just savings and it was all for money he saved. I mean, giving kids money is pretty dangerous.
I guess you see this, but if he has income (you hire him to take the trash out?) and you make it a Roth IRA, it'll be a Roth IRA and all that goes with that.
If you make it a coverdell or 529, it'll be a coverdell or a 529 and all that goes with that.